In web and coating manufacturing operations, there is a need for an apparatus for accurate, on-line measurements of web and coating layer thickness. Such an apparatus should have a fast response time and a high degree of lateral resolution; and should be portable, light weight, compact, and easy to set up. Further, there is a need for such an apparatus which can be readily installed in high and low temperature environments, in the presence of solvents, high air flow, and various levels of relative humidity. In addition, there is a need for such an apparatus which is self-calibrating or able to remain in calibration for extended periods of time so that the apparatus can be installed on a production machine without the need for re-calibration.
Driving forces for this invention are the needs to achieve improved manufacturing process understanding and perform on-line manufacturing process capability assessments in a minimum amount of time. A high level of portability is needed to enable short turn-around time during troubleshooting activities for production quality problems.
The invention relates to the measurement of physical properties of an object by optical interferometry. Accordingly, a brief background discussion is presented regarding non-coherent and coherent light interferometers.
The coherence length of a light source is the distance over which the phase relationships of a light beam originating from the light source remain correlated. For a coherent light source with a long-coherence length, such as a helium-neon laser, this distance can be many kilometers. In contrast, for a non-coherent broadband white-light source such as sunlight, this distance is only a few micrometers (microns). For example, typical broadband light emitting diodes (LED) have coherence lengths on the order of 8-15 .mu.m.
FIG. 1 shows a block diagram of a prior art fiberoptic embodiment of a non-coherent light interferometer 10 in a Michelson configuration. Non-coherent interferometer 10 has a non-coherent light source 14 emitting a non-coherent light signal. The non-coherent light signal from non-coherent light source 14 is coupled into a single-mode optical fiber 16. Single-mode optical fiber 16 comprises one arm of a 2.times.2 optical coupler 18 used as a splitting means. Optical coupler 18 divides the non-coherent light signal from non-coherent light source 14 traveling along single-mode optical fiber 16 into first and second light signals, of substantially equal intensity, traveling along single-mode optical fibers 20 and 22, respectively. The first light signal traveling along single-mode optical fiber 20 is incident on an applying and collecting means 24. The first light signal is coupled to an object 25 through applying and collecting means 24. A portion of the first light signal is then reflected back from object 25 into single-mode optical fiber 20 through applying and collecting means 24. The second light signal, traveling along single-mode optical fiber 22 is incident on a collimating applying and collecting means 26. This second light signal is collimated by collimating applying and collecting means 26 and the collimated second light signal is directed towards an optical element 28, such as a retroreflector, mirror, or a combination of the two. A portion of the collimated light originating from the second light signal is reflected back from optical element 28 and is coupled back into single-mode optical fiber 22 by collimating applying and collecting means 26. Optical element 28 is mounted onto an actuation means 30, such as a motorized translation stage, a driven screw mechanism, or a voice-coil actuator (hereinafter called motor 30), which provides for precision movement of optical element 28 in a direction shown by an arrow A. The speed of motor 30 is controlled accurately so that when optical element 28 is reciprocated, the velocity of optical element 28 is a constant during the time intervals at which measurements are performed (i.e., a measurement cycle).
In operation, the first and second light signals traveling along single-mode optical fibers 20 and 22, respectively, are reflected back to optical coupler 18 where they recombine to form a recombined light signal, and a portion of the recombined light signal is directed into a photodetector 32 by a single-mode optical fiber 34. Photodetector 32 is used to measure the recombined light signal strength as a function of the displacement of optical element 28.
Non-coherent light interferometer 10 includes four branches: a non-coherent light source branch, an object branch, a reference branch, and a detection branch. The light source branch comprises the path from non-coherent light source 14 to optical coupler 18. The total optical path from optical coupler 18 to object 25 and back to optical coupler 18 is defined as the optical path length of the object branch of noncoherent light interferometer 10. Similarly, the total optical path from optical coupler 18 to optical element 28 and back to optical coupler 18 is defined as the optical path length of the reference branch of non-coherent light interferometer 10. Likewise, the path from optical coupler 18 to photodetector 32 is defined as the detection branch of non-coherent light interferometer 10. The object branch and the reference branch are referred to as the interfering branches of non-coherent light interferometer 10.
During operation of non-coherent light interferometer 10, motor 30 drives optical element 28 closer and further away from collimating applying and collecting means 26. As optical element 28 is moved closer and further, the optical path length of the reference branch is varied. This scanning interrogates different depths of object 25 as optical element 28 is traversed in either direction.
Since non-coherent light source 14 has a short coherence length, constructive interference in the recombined light signal occurs only when the light signals in the object branch and reference branch of interferometer 10 are mutually coherent. Mutual coherence occurs when the object branch and reference branch are of nearly equal optical path length. The maximum magnitude of the constructive interference signal occurs when the optical path lengths of the object branch and reference branch are equal. A series of interference fringes will be observed as optical element 28 is scanned through the region of mutual coherence. The intensity of these interference fringes will vary from zero to a maximum over a few coherence lengths of the light source.
For constructive interference to occur, there also needs to be a reflected light signal coming from object 25 which is coupled back into single-mode optical fiber 20. To obtain a reflected light signal coming from object 25 at a given interrogation depth, there must be an optical interface between adjacent optical media of object 25 with differing group index of refraction. This can occur, for example, at a fiber-optic connector-to-air interface, the air-to-object front surface interface, and the object second surface-adjacent-media interface (if one exists). Constructive interference will thus be observed at positions of optical element 28 wherein the optical path length of the reference branch is equal to the optical path length of the object branch within a few coherence lengths for each of the object's optical media interfaces of differing group index of refraction.
Referring still to FIG. 1, optical element 28 is mounted for precision movement by motor 30 in a direction shown by the arrow A. Constant-velocity control of motor 30 is utilized to obtain accurate measurements. Use of constant velocity enables calculation of motor distance traveled from measured time intervals. Constant velocity is typically obtained by means of an active servo control loop which generates a velocity versus time graph for optical element 28 (and accordingly for motor 30), such as that shown in FIG. 2. The acceleration and deceleration phases at each end of the measurement cycle are variable-velocity zones where accurate measurements cannot be made. The acceleration phase occurs between points a and b, the constant-velocity phase is shown between points b and c, and the deceleration phase occurs between points c and d. In practice, it is difficult to minimize velocity variations between points b and c to the desired degree. This difficulty limits the precision of the instrument. Since there is also the possibility for hysteresis and backlash accumulation, measurements are usually made during travel in one direction only. A typical measurement cycle would be as follows. A measurement is performed when optical element 28 (as illustrated in FIG. 1) is moving from left to right during part of the constant-velocity phase b to c. In practice, a home reference position e (as shown in FIG. 2) would also be detected in order to ensure that the constant-velocity phase b to c has been reached. FIG. 2 shows the time at which the motor crosses the home reference position e, labeled as point e. This location of point e would vary from scan to scan due to hysteresis and backlash of motor 30. The measurement cycle would be started when the motor passes home reference position e while traveling from left to right.
FIG. 3 shows a block diagram of a typical Michelson based coherent light interferometer 40. Interferometer 40 includes a coherent light source 42, such as a laser, emitting a collimated coherent light signal. The coherent light signal emitted from coherent light source 42 is split at point B of a splitting means 44, such as beam splitter, into first and second light signals of approximately equal intensity. The first light signal is incident onto a stationarily mounted retroreflector 46. The second light signal is incident onto an optical element 48, such as a retroreflector (hereinafter referred to as retroreflector 48), mounted for precision movement by motor 30 in a direction shown by arrow A. The first and second light signals are retro-reflected back to beam splitter 44 where they recombine at point C and interfere with each other. This recombined, interference signal is detected by a photodetector 50.
Coherent light interferometer 40 includes four branches: a coherent light source branch, a stationary branch, a reference branch, and a detection branch. The total optical path from point B of beam splitter 44 to retroreflector 46 and back to point C of beam splitter 44, is defined as the optical path length of the stationary branch of coherent light interferometer 40. Similarly, the total optical path from point B of beam splitter 44 to retroreflector 48 and back to point C of beam splitter 44, is defined as the optical path length of the reference branch of coherent light interferometer 40. The coherent light source branch and detection branch of the coherent light interferometer follow the definitions for the light source branch and detection branch, respectively, of non-coherent light interferometer 10.
Since coherent light source 42 has a long coherence length, the stationary branch and the reference branch of coherent light interferometer 40 need not have equal optical path lengths in order for an interference signal to be detected at photodetector 50. Since this is the case, interference fringes of equal amplitude will be observed over the entire range of motion of retroreflector 48.
The configuration shown in FIG. 3 can be used for a bulk non-coherent light interferometer by changing the coherent light source to a non-coherent light source, and making the path lengths of the stationary branch and reference branch equal to within a few coherence lengths of the non-coherent light source. When the stationary retroreflector, i.e. retroreflector 46, of FIG. 3 is replaced with object 25 (as shown in FIG. 1) and the path lengths of the stationary and reference branches are nearly equal, the configuration then becomes functionally equivalent to the fiber-optic implementation of the non-coherent light interferometer 10, as shown in FIG. 1.
Non-coherent light interferometers have been demonstrated to generate an optical fringe pattern that can be utilized to determine a predetermined physical property, such as thickness, of a traveling web. For example, U.S. Pat. No. 3,319,515 (Flournoy) relates to the determination of a physical property of a substance on the basis of interferometric optical phase discrimination. However, as described in U.S. Pat. No. 4,958,930 (Robertson, Jr), the measurement system described by Flournoy does not provide a mechanism whereby small variations (in the range of less than one percent) in thickness of traveling webs and coatings can be accurately gauged
while on-line; that is, without removing the web or coated layer from the manufacturing line. In addition, the apparatus described in Flournoy is bulky, difficult to set up, and can not be readily utilized in many spatially constrained process environments.
The measurement approach described in Flournoy has come to be known as "Optical Coherence-Domain Reflectometry" (OCDR) as exemplified by the following articles: (1) "Optical Coherence-Domain Reflectometry: a New Optical Evaluation Technique", by Robert C. Youngquist, Sally Carr, and D. E. N. Davies, Optics Letters, Vol. 12, No. 3, March 1987, pp. 158-160; (2) "Guided-wave Reflectometry with Micrometer Resolution", by B. L. Danielson and C. D. Whittenberg, Applied Optics, Vol. 26, No. 14, Jul. 15, 1987, pp. 2836-2842; (3) "Polarization-Independent Interferometric Optical-Time Domain Reflectometer", by Masaru Kobayashi, Hiroaki Hanafusa, Kazumasa Takada, and Juichi Noda, Journal of Lightwave Technology, Vol. 9, No. 5, May 1991, pages 623-628; (4) "Design of a Precision Optical Low-Coherence Reflectometer", by D. H. Booster, H. Chou, M. G. Hart, S. J. Mifsud, and R. F. Rawson, Hewlett-Packard Journal, Vol. 44, No. 1, February 1993, pages 39-43; and (5) "High-Resolution and High-Sensitivity Optical Reflection Measurements Using White-Light Interferometry", by H. Chou and W. V. Sorin, Hewlett-Packard Journal, Vol. 44, No. 1, February 1993, pages 52-59. U.S. Pat. No. 5,202,745 (Sorin) also describes a measurement system based on the OCDR technique. Each article describes a reference mirror in one arm of a Michelson interferometer which is scanned at a constant velocity during the course of measurement. Measurement resolution on the order of a few microns has been reported. The ultimate measurement resolution attainable by this class of instruments is dependent on how precisely velocity variations can be minimized while changing the path length of the reference arm of the interferometer. The ultimate measurement resolution is also dependent on the methods used to process photodetector 32 data.
The above identified references have achieved certain degrees of success in their particular applications. However, a need has continued to exist for a high precision, compact, portable and robust apparatus for determining physical properties of an object, for example, during high-speed manufacturing of running webs and/or coatings. More specifically, there is a need for an on-line apparatus capable of high-speed thickness gauging of liquid layers and web material.